1. Field of the Invention
The present invention relates to a film thickness measuring apparatus and more particularly to improvement of or relating to an apparatus for measuring film thickness of material with the use of spectral reflectivity.
2. Description of the Prior Art
It is observed that when monochromatic light generated by the monochromator is emitted toward the surface of the film structure of the material of which film thickness is to be measured, reflection interference takes place in dependence on wavelength of the introduced light beam. Since there is existent a specific relation between the number of fringes each of which includes a bright portion (or a dark portion) and the thickness of the film structure, it is possible to measure the thickness of a film by counting the number of fringes relative to wavelength of employed light beam.
As is well known, a film thickness measuring apparatus of the above-mentioned type has spectral intensity distribution as typically illustrated in FIG. 3 in which abscissa represents wavelength in nm and ordinate does reflection intensity. Spectral intensity distribution for light source, diffraction grating and photomultiplier tube constituting the film thickness measuring apparatus are exemplified in the range of 400 nm to 800 nm in the drawing. As is apparent from the drawing, performance of the light source extends in the form of a curve 10 which rises at a steep inclination angle in the region of short wavelength on the assumption that a tungsten lamp is employed for the light source. On the other hand, spectral intensity distribution of the diffraction grating is represented in the form of a curve 12 which extends downwardly in the region located in the vicinity of both ends of wavelength. Further, a curve 14 represents output performance of photomultiplier tube. As is apparent from the drawing, the curve 14 extends downwardly at a steep inclination angle in the region where light beam has long wavelength. Accordingly, spectral intensity distribution finally detected by the photomultiplier tube is represented by compound spectral intensity distribution as represented by a curve 16 which is constituted by a combination of those of light source, diffraction grating and photomultiplier tube as illustrated by the curves 10, 12 and 14. The compound spectral intensity distribution curve 16 represents a curve which has lower sensitivity in both the regions, one of them being such that light beam has longer wavelength and the other one being such that it has shorter wavelength, but it has higher sensitivity in the intermediate region. Namely, the curve 16 extends in the form of a curve having the convex configuration. Further, the curve 16 is superimposed with spectral reflection spectrum curve 18 which is obtained by a number of measurements of film thickness of the material.
However, it has been found that the conventional film thickness measuring apparatus utilizing compound spectral intensity distribution as described above has a drawback that measuring accuracy of spectral reflection spectrum decreases remarkably in the region located in the vicinity of 400 nm and 800 nm where the curve extends downwardly at a steep inclination angle. Namely, the apparatus has a problem that peak and dip on the curve are discriminated only with reduced accuracy. This means that the conventional apparatus carries out measurements of film thickness with undesirable inaccuracy.